This invention relates to laminar blends of polyolefins and ethylene-vinyl alcohol copolymers which have superior barrier and physical properties.
Containers and films which are blends or laminates of polyolefins and ethylene-vinyl alcohol copolymers have been the subject of several investigations because this combination of polymers has the potential for providing high resistance to permeation by water, oxygen, and hydrocarbons.
U.S. Pat. No. 3,975,463 discloses a resinous composition of (a) 30 to 98% by weight of a crystalline polyolefin, (b) 2 to 70% by weight of a saponified product of an ethylene vinyl acetate copolymer, and (c) 0.5 to 15 parts by weight of a thermoplastic polymer containing a carbonyl group in the main or side chain, and a molded structure formed therefrom, which has a specific multilayer structure and a high gas permeation resistance. The thermoplastic polymer (c) can be a copolymer of maleic anhydride or an acrylic acid grafted polyethylene.
European Patent Application 15556 discloses a process for manufacturing laminar articles by forming heterogeneous melts of a combination of a polyolefin and a second polymer with an alkylcarboxyl substituted polyolefin as a compatibilizing material, and the laminar articles so formed. The second polymer can be poly(ethylene-co-vinyl alcohol), and the compatibilizing material can be obtained by melt-grafting fumaric acid onto polyethylene. In particular, a grafted material is disclosed which was grafted with about 0.9 weight percent fumaric acid; the ratio of polyolefin:second polymer:compatibilizer can be 65:30:5.
A study of barrier materials prepared following the teachings of U.S. Pat. No. 3,975,463 and European Patent Application 15556 revealed that the laminar structures of European Patent Application 15556 are capable of providing better barriers than the composites of U.S. Pat. 3,975,463. However, the laminar structures prepared following the teaching of European Patent Application 15556 (particularly Examples 12 and 13) were found to exhibit significant variation in resistance to permeation by hydrocarbons and oxygen and in mechanical strength. Such variations are undesirable in laminar structures which are to be used in demanding applications such as containers for hydrocarbon fuels and packaging for perishable foodstuffs. Thus a need exists to provide polyolefin/ethylene vinyl alcohol copolymer laminar structures which will consistently provide optimum barrier properties and mechanical strength.